The present invention relates to a cooling mixer for bulk material, especially plastic material, in powder and granular form that comprises a cylindrical container with a container double wall and at least one outlet, a rotating mixing device comprised of a hub and mixing paddles fixedly connected to the hub, the mixing device concentrically connected to the container such that a vertical rotation axis of the mixing device coincides with the axis of the container, a coolant circuit containing a coolant that is circulated during operation of the cooling mixer within an intermediate space of the container double wall, and a cover for closing the container having at least one feed opening.
Cooling mixers of the aforementioned kind are especially employed for cooling plastic materials that have been prepared in heating mixers for preventing an undesired change of the properties, generated in the heating step, due to excessive temperatures.
Known cooling mixers having a mixing device rotating about a vertical rotating axis have container double walls whereby the container double walls during operation of the cooling mixer are cooled by a coolant, preferably water, and have a minimum height which corresponds approximately to the diameter of the container.
In order to be able to at least approximately use the entire mantle surface area of the cooling mixer for cooling the bulk material, the mixing device is operated at high circumferential speeds whereby, due to friction within the bulk material and between bulk material and mixer, the mixing chamber is provided with additional heat and whereby the inner area of the container bottom only insufficiently contributes to the cooling of the bulk material due to centrifugal forces acting on the bulk material.
It is furthermore known to provide coolable inserts to the interior of the container in order to increase the cooling surface area.
However, such inserts obstruct the forced circulation and mixing of the bulk material generated by the mixing device which results in a slower change of position of the particles of the bulk material and thus in an insufficient use of the cooling surfaces, especially of the inner container mantle surface. Also, due to the friction of the bulk material at the inserts, additional heat is generated within the mixing chamber.
According to German Patent 19 56 241 an annular double wall insert is provided which with its shape and arrangement within the container is adapted to the forced circulation of the bulk material and thus provides a uniform contact of the bulk material at the additional cooling surfaces; however, its contribution to the reduction of friction at the additional cooling surfaces and to the effective use of the other cooling surfaces is neglectable.
It is therefore an object of the present invention to provide a cooling mixer of the aforementioned kind such that a better use of the cooled surfaces due to a faster change of position of the particles to be mixed is achieved without generating a substantial amount of frictional heat within the mixing chamber.